The development of highly absorbent members for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harnon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter "hydrogel-forming absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
Prior to the use of these hydrogel-forming absorbent polymers, it was general practice to form absorbent structures, such as those suitable for use in infant diapers, entirely from wood pulp fluff. Given the relatively low amount of fluid absorbed by wood pulp fluff on a gram of fluid absorbed per gram of wood pulp fluff, it was necessary to employ relatively large quantities of wood pulp fluff, thus necessitating the use of relatively bulky, thick absorbent structures. The introduction of these hydrogel-forming absorbent polymers into such structures has allowed the use of less wood pulp fluff. These hydrogel-forming absorbent polymers are superior to fluff in their ability to absorb large volumes of aqueous body fluids, such as urine (i.e., at least about 15 g/g), thus making smaller, thinner absorbent structures feasible.
These hydrogel-forming absorbent polymers are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like. These polymers are rendered water-insoluble, yet water-swellable, by slightly cross-linking the carboxyl group-containing polymer chains with conventional di- or poly-functional monomer materials, such as N, N'-methylenebisacrylamide, trimethylol propane triacrylate or triallyl amine. These slightly crosslinked absorbent polymers still comprise a multiplicity of anionic (charged) carboxyl groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb body fluids as the result of osmotic forces, thus forming hydrogels.
The degree of cross-linking determines not only the water-insolubility of these hydrogel-forming absorbent polymers, but is also an important factor in establishing two other characteristics of these polymers: their absorbent capacity and gel strength. Absorbent capacity or "gel volume" is a measure of the amount of water or body fluid that a given amount of hydrogel-forming polymer will absorb. Gel strength relates to the tendency of the hydrogel formed from these polymers to deform or "flow" under an applied stress. Hydrogel-forming polymers useful as absorbents in absorbent structures and articles such as disposable diapers need to have adequately high gel volume, as well as adequately high gel strength. Gel volume needs to be sufficiently high to enable the hydrogel-forming polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength needs to be such that the hydrogel formed does not deform and fill to an unacceptable degree the capillary void spaces in the absorbent structure or article, thereby inhibiting the absorbent capacity of the structure/article, as well as the fluid distribution throughout the structure/article. See, for example, U.S. Pat. No. 4,654,039 (Brandt et al), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Reissue Pat. No. 32,649) and U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
Prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989 (preferably from about 9 to about 50% hydrogel-forming absorbent polymer in the fibrous matrix). There are several reasons for this. The hydrogel-forming absorbent polymers employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in "gush" situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.
More importantly, many of the known hydrogel-forming absorbent polymers exhibited gel blocking, especially when included in the absorbent structure at higher levels. "Gel blocking" occurs when particles of the hydrogel-forming absorbent polymer are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of hydrogel-forming absorbent polymer in the absorbent member are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent member.
Gel blocking can be a particularly acute problem if the particles of hydrogel-forming absorbent polymer do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. Low gel strength hydrogel-forming absorbent polymers also tend to be those having higher fluid capacities. Gel strength can be increased by surface crosslinking of these higher fluid capacity hydrogel-forming absorbent polymers. Unfortunately, while surface crosslinking increases gel strength, it also tends to lower the fluid capacity of the hydrogel-forming absorbent polymer.
Gel blocking can also occur when the hydrogel-forming absorbent polymer is in the form of regular shaped particles, such as spherically shaped particles. Spherical shaped particles typically result when the hydrogel-forming absorbent polymer is formed by multi-phase polymerization processing techniques such as inverse emulsion polymerization or inverse suspension polymerization procedures. See U.S. Pat. No. 4,340,706 (Obaysashi et al), issued Jul. 20, 1982, U.S. Pat. No. 4,506,052 (Flesher et al), issued Mar. 19, 1985, and U.S. Pat. No. 4,735,987 (Morita et al), issued Apr. 5, 1988. Because these spherical shaped particles are prone to gel blocking, multi-phase polymerization processing techniques have been considered less desirable in synthesizing hydrogel-forming absorbent polymers. See U.S. Pat. No. 5,124,188 (Roe et al), issued Jun. 23, 1992.
To improve capillary capability and thus minimize gel blocking, particles of these hydrogel-forming absorbent polymers have been formed into interparticle crosslinked aggregate macrostructures, typically in the form of sheets or strips. These aggregate macrostructures have been prepared by initially mixing the particles of hydrogel-forming absorbent polymer with a solution of a nonionic crosslinking agent such as glycerol, water and a hydrophilic organic solvent such as isopropanol. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992; U.S. Pat. No. 5,124,188 (Roe et al), issued Jun. 23, 1992; and U.S. Pat. No. 5,149, 344 (Lahrman et al), issued Sep. 22, 1992. See also U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994, which discloses an improved porous aggregate macrostructure where the particles of hydrogel-forming absorbent polymer are crosslinked with cationic amino-epichlorohydrin adducts, such as KYMENE.
Because the particulate nature of the absorbent polymer is retained, these macrostructures provide pores between adjacent particles that are interconnected such that the macrostructure is fluid permeable (i.e., has capillary transport channels). Due to the interparticle crosslink bonds formed between the particles, the resultant macrostructures also have improved structural integrity, increased fluid acquisition and distribution rates, and minimal gel blocking characteristics. Even so, the fluid handling capability of these macrostructures is still somewhat dependent on the fluid handling capability of the particles of hydrogel-forming absorbent polymer from which they are made. For example, macrostructures made from low gel strength hydrogel-forming absorbent polymers or spherical shaped particles of hydrogel-forming absorbent polymer are still potentially subject to gel blocking. Also, macrostructures made from surface crosslinked hydrogel-forming absorbent polymers still have less than optimum permeability characteristics.
Accordingly, it would be desirable to be able to make absorbent aggregate macrostructures of bonded absorbent particles that: (1) can use hydrogel-forming absorbent polymers made by a variety of methods; (2) are less prone to gel blocking; (3) have optimum permeability characteristics; and/or (4) provide an improved combination of fluid handling capabilities.